I. Field of the Disclosure
The technology of the disclosure relates generally to battery monitoring systems for monitoring the state-of-health (SOH) of battery cells in battery power systems, such as uninterrupted power supplies (UPSs).
II. Background
An industrial system may rely on an uninterrupted power supply (UPS) to provide backup power in the event of failure of a primary power system. The UPS may be provided in the form of a number of lead acid battery cells electrically connected in series. A battery charger is provided that keeps the battery cells charged in the event backup power is needed from the battery cells. However, each battery cell will eventually fail. For example, lead acid batteries may lose the ability to accept a charge when discharged over time due to sulfation. A battery containing one or more failed battery cells may be unable to power the industrial system at specified battery operating voltages, at specified battery operating currents, and/or for specified battery time durations.
Accordingly, an industrial system may employ a battery monitoring system to monitor the state-of-health (SOH) of battery cells in a battery power supply system. The SOH of a battery cell may be correlated with an ohmic value of the battery cell, such as an internal resistance, internal impedance, and/or internal conductance of the battery cell. For example, an increased internal resistance, increased internal impedance, and/or decreased internal conductance of a battery cell may be used to detect an impending failure or failure of the battery cell. In this regard, a battery cell that has been detected to have the impending failure or to have failed may be replaced.
In this regard, FIG. 1 illustrates a battery monitoring system 10. The battery monitoring system 10 comprises a battery monitoring device 14 and a battery monitoring system control unit 12 for controlling the battery monitoring device 14. The battery monitoring device 14 is configured to test ohmic values 16 of battery cells 18 of a battery power supply system provided in the form of a battery 20. The battery 20 of the battery power supply system may be used to provide a backup battery supply, such as an uninterrupted power supply (UPS), to supply power to power-consuming components or systems. The battery 20 is comprised of a plurality of battery cells 18(1)-18(4) electrically connected in series. Each battery monitoring device 14 may be coupled to a subset 22 of the battery cells 18(1)-18(4) electrically connected in series and in a sequential order to form the battery 20. The subset 22 of battery cells 18(1)-18(4) may comprise a battery cell substring comprising a unique set of battery cells 18 of the battery 20. The battery monitoring device 14 provides a pair of current-inducing leads L1, L2 configured to be coupled to the negative and positive terminals of a battery cell substring of the battery 20. The battery monitoring system control unit 12 instructs the battery monitoring device 14 to produce a current through the subset 22 of battery cells 18(1)-18(4) (i.e., one or more of the battery cells 18(1)-18(4)) by activating a switch to place a resistive load in a current loop with the subset 22 of battery cells 18(1)-18(4)) of the battery 20.
The battery monitoring device 14 also provides a plurality of voltage sensing leads V1-V5. The voltage sensing leads V1-V5 are configured to be coupled to measure a voltage across the negative and positive terminals of each battery cell 18(1)-18(4). As illustrated in FIG. 1, voltage leads V1-V5 have resistances RV1-RV5 and current leads L1-L2 have resistances RL1-RL2. To increase the accuracy of measured voltages, the battery monitoring device 14 may employ Kelvin sensing. In this regard, separate voltage leads V1, V5 (for sensing voltages) may optionally be provided separate from the current leads L1, L2. Providing the current leads L1, L2 separate from the voltage leads V1-V5 allows the voltages measured by the voltage leads V1-V5 to be more accurate than a system in which a single lead is used for both L1 and V1 and another single lead is used for both L2 and V5. This is because separating the current lead L1 from the voltage lead V1 and separating the current lead L2 from the voltage lead V5 significantly reduces the impedance contribution of the voltage leads V1, V5. Because there is almost no current flow in the voltage leads V1, V5, the voltage drop across the voltage leads V1, V5 (i.e., across RV1 and RV5) is lower. As a result, using separate current leads L1, L2 and voltage leads V1, V5 enables a more accurate measurement of the voltages across the battery cells 18(1)-18(4).
To determine the SOH of a tested battery cell 18 among a subset 22 of the battery cells 18(1)-18(4), an assigned battery monitoring device 14 determines an ohmic value 16 of the tested battery cell(s) 18. The ohmic value 16 of the tested battery cell(s) 18 is determined by placing a current load across the tested battery cell(s) 18 to place an effective load on the tested battery cell(s) 18. The current load causes the tested battery cell(s) 18 to draw a predetermined amount of current from the tested battery cell(s) 18. The resulting voltage across the tested battery cell(s) 18 is then measured to determine to ohmic value 16 of the tested battery cell(s) 18 according to Ohm's law, to determine the SOH of the tested battery cell(s) 18.